In the field of radio frequency (RF) communications, it is often desirable to be able to focus, direct, or otherwise manipulate an RF signal. Traditionally this has been accomplished by placing a reflective surface in the signal path, either to gather and focus a signal being received or to concentrate a transmitted signal. While flat surfaces reflect RF energy, their effect is very much like an optical mirror in that they reflect an incident signal at an orthogonal angle to the angle of incidence and, consequently, perform no concentrating or focusing function. The use of a curved (e.g., a parabolic) surface, however, does provide a concentrating, focusing function.
The use of satellite communications has increased the demand for circularly polarized antennas and for dual polarization antennas. For instance, many of the satellite transponders in use today carry two programs on the same frequency by using separate polarizations. Thus, a single antenna structure may be called upon to simultaneously receive two polarizations, or perhaps to transmit in one polarization and receive in another. The single antenna structure therefore separates the two polarization channels, to a high degree of isolation.
It is possible to have dual linear or dual circular polarization channel diversity. That is, a frequency may be reused if one channel is vertically polarized and the other horizontally polarized. Or, a frequency can also be reused if one channel uses right hand circular polarization (RHCP) and the other left hand circular polarization (LHCP). Polarization refers to the orientation of the E field in the radiated wave, and if the E field vector rotates in time, the wave is then said to be rotationally or circularly polarized.
An electromagnetic wave (and radio wave, specifically) has an electric field that varies as a sine wave within a plane coincident with the line of propagation, and the same is true for the magnetic field. The electric and magnetic planes are perpendicular and their intersection is in the line of propagation of the wave. If the electric-field plane does not rotate (about the line of propagation) then the polarization is linear. If, as a function of time, the electric field plane (and therefore the magnetic field plane) rotates, then the polarization is rotational. Rotational polarization is in general elliptical, and if the electric field vector extremity describes a circle over time then the polarization is circular. The polarization of a transmitted radio wave is determined in general by the transmitting antenna (and feed)—by the type of the antenna and its orientation. For example, the monopole antenna and the dipole antenna are two common examples of antennas with linear polarization. An axial mode helix antenna is a common example of an antenna with circular polarization, and another example is a crossed array of dipoles fed in quadrature. Linear polarization is usually further characterized as either Vertical or Horizontal. Circular Polarization is usually further classified as either Right Hand or Left Hand.
The dipole antenna has been perhaps the most widely used of all the antenna types. It is of course possible however to radiate from a conductor which is not constructed in a straight line. Preferred antenna shapes are often Euclidian, being simple geometric shapes known through the ages. In general, antennas may be classified as charge separation or charge conveyance types, corresponding to dipoles and loops, and line and circle structures.
Radiation can occur from 3 complimentary forms of the same geometry: panel antennas, slot antennas and skeleton antennas. In dipoles, these can correspond to a flat metal strip, a straight slot cut out of a flat metal sheet, or a rectangle of wire. Thus, the same antenna geometry may be reused in accordance with Babinet's Principle.
Circular polarization for dipole antennas has been attributed to George Brown, which was described in the literature as “The Turnstile Antenna”, Electronics, 9, 15, Apr. 1936. In the dipole turnstile, crossed orthogonal dipoles are fed in phase quadrature: 0, 90 degrees at the dipole ports. The phases at the dipole terminals are 0, 90, 180, and 270 degrees from each other at all times.
Approaches to circular polarization in loop antennas appear lesser known, or perhaps even unknown in the purest forms. For instance, the present edition “Antenna Engineering Handbook”, R. Johnson and H. Jasik editors, does not describe methods to obtain circular polarization from a single loop antenna. In spite of the higher gain of the full wave loop vs. the half wave dipole (3.6 dBi vs. 2.1 dBi), dipoles are commonly used for circular polarization needs, as for instance in turnstile arrays. Both the dipole turnstile and a single loop antenna are planar, in that their thin structure lies nearly in a single plane.
While many structures are described as loop antennas, the canonical loop shape is that of a circle. The resonant loop is a full wave circumference circular conductor, often called a “full wave loop”. The typical prior art full wave loop is linearly polarized, having a radiation pattern that is a two petal rose, with two opposed lobes normal to the loop plane, and a gain of about 3.6 dBi. Plane reflectors are often used with the full wave loop antenna to obtain a unidirectional pattern.
Polarization diversity has commonly been obtained from crossed dipole antennas. For instance, U.S. Pat. No. 1,892,221, to Runge, proposes a crossed dipole system with the dipoles fed at 0 and 90 degree phasing. Although circular polarization resulted, only polarization diversity was described.
U.S. Pat. No. 6,522,302 to Iwasaki and entitled “Circularly-Polarized Antennas” is directed to a circularly polarized antenna array rather than a single circularly polarized loop element. A circle is among the most elemental of antenna structures, and may be the most fundamental single geometry capable of circular polarization.
Communication satellites are in widespread use for communicating data, video and other forms of information between widely spaced locations on the earth's surface. Antennas are transducers between transmission lines and free space. A general rule in antenna design is that, to direct or “focus” the available energy to be transmitted into a narrow beam, a relatively large “aperture” is necessary. The aperture may be provided by a broadside array, a longitudinal array, or an actual physical aperture such as the mouth of a horn.
Another type of antenna is a reflector antenna, which in a receive mode, receives a collimated beam of energy and focuses the energy into a converging beam directed toward a feed antenna, or which, in a transmit mode, focuses the diverging energy from a feed antenna into a collimated beam. Those skilled in the art know that antennas are reciprocal devices, in which the transmitting and receiving characteristics are equivalent. Generally, antenna operation is referred to in terms of either transmission or reception, with the other mode being understood therefrom. A conventional reflector antenna 10, e.g. as shown in FIG. 1, may include a feed 12 and a dish 14, such as a parabolic dish, for focusing the energy.
U.S. patent application Ser. No. 11/609,046 entitled “Multiple Polarization Loop Antenna And Associated Methods” to Parsche et al. includes methods for circular polarization in loop antennas. A full wave circumference loop is fed in phase quadrature (0°, 90°) using two driving points.
U.S. Pat. No. 3,122,745 to Ehrenspeck is entitled “Reflection Antenna Employing Multiple Director Elements And Multiple Reflection Of Energy To Effect Increased Gain” is directed towards “backfire” antennas. A slow wave antenna, such as a yagi uda is pointed towards a plane reflector, for the enhancement of gain and the reduction of sidelobes. This was perhaps counterintuitive to common practice, as director elements of yagi-uda antennas are often towards the direction of communications. Backfire antennas are further described in “The Short Backfire Antenna”, Proceedings Of the IEEE, 53, 1138-1140, August 1965.
U.S. Pat. No. 4,017,865 to Woodward is entitled “Frequency Selective Reflector System” and is directed to a dual-band Cassegrain antenna system. The antenna system includes a main parabolic reflector and a hyperbolic subreflector that reflects signals at a first band of frequencies and transmits signals at a second lower band of frequencies. The hyperbolic subreflector according to one embodiment is a square grid mesh with conductive rings centered along the connecting legs of the square grid mesh.
U.S. Pat. No. 6,198,457 to Walker, et al. is entitled “Low-wind Load Satellite Antenna” and is directed to a satellite communications antenna that includes a low-wind load reflector so that the antenna may be used on high wind load locations, such as on a ship. The reflector has a support structure which includes a grid-like structure having relatively large apertures therein to allow wind to pass therethrough. Unlike solid surfaced parabolic reflectors, the reflector in Walker et al. includes reflective radiating elements, such as dipoles, mounted to the support structure for focusing at least one desired frequency of operation.
The reflector in Walker et al. is designed to have low wind drag and is based upon the premise that any surface shape can be designed to electromagnetically act as though it were a parabolic reflector. A more detailed description of this concept is provided in U.S. Pat. No. 4,905,014 to Gonzalez et al., the disclosure of which is incorporated herein by reference and which is commonly referred to in the industry as FLAPS™ (Flat Parabolic Surface) technology, e.g. as illustrated in FIG. 2. The antenna 20 includes a feed 22 and reflector 24, and the effect is achieved by introducing appropriate phase delays at discrete locations along the reflector surface. In-phase combining occurs at the array “focus” due to the tuning of individual reflector elements. A typical implementation of the concept includes an array of shorted dipole scatterers 26 positioned above a ground plane or above a reflecting shorted dipole.
However, there is still a need for a low wind load satellite communications antenna with more gain at a reduced size, in the interests of convenience, utility and cost.